Ignore the lines on your cutting mat and use the ruler exactly like the video shows. I have my mat upside down - I like the plain side better. If all fails, look for the June Tailor Shape Cut. You still have to square the fabric and measure carefully, but it does keep the cutter going straight and makes it easy to cut repeats without repostioning.
If your daughter wants purple, then she shall have purple. Don't sweat the little stuff if she in the teen or preteen years and that is little stuff.